This invention pertains to hollow, thick walled macrospherical particles intended for use primarily as anti-perspirants. The macrospherical particles may also be used in pigments, resins, catalysts, etc.
For the last 10 or 15 years, aerosol sprays have been a major application form for many products such as hair spray, paint, anti-perspirant powders, and countless others. For the purposes of this application, "aerosol" means a suspension of fine solid particles in a gas. The gas need not be halohydrocarbons such as Freon which have been widely used as propellents, but may include air or any other gaseous propellent.
In a recent article, Cambridge, G.W., "Inhalation Toxicity Studies", Aerosol Age, May 1973, 32, the author focused on the current public and regulatory awareness to pulmonary deposition and possible retention of inhaled aerosol products. The study pointed out that, while penetration into and deposition in the respiratory tract is influenced to some extent by the frequency and depth of breathing, the major factor is the size and shape of the particle representing inhaled particulate matter. The nose, as the primary filter, retains virtually all particles in excess of 10 microns in diameter. Approximately 50% of 5 micron particles are retained, while almost all 1 - 2 micron particles penetrate beyond the nose. Particles below 5 microns are respirable and will penetrate into the lung if the particles are of unit or lower density.
Hatch, T. F. and Gross, P., Pulmonary Deposition and Retention of Inhaled Aerosols, Academic Press, N.Y., 1964, define aerodynamic particle size as "the diameter of a unit density sphere having the same settling velocity as a particle in question of whatever shape and density". These authors have shown that the degree of respiratory penetration and retention is a direct function of the aerodynamic particle size. In effect, the denser a particle, the less respirable it is, even at particle diameters less than 10 microns, if the density is a high order of magnitude.
Sciarra, J.J., McGinley, P., and Izzo, L., "Determination of Particle Size Distribution of Selected Aerosol Cosmetics. I. Hair Sprays", J. Soc. Cosm. Chem. 20, 385-394, May 27, 1969, reported that while most particles below 50 microns will remain suspended in air for relatively long periods of time, only those particles less than 10 microns are likely to pass into the respiratory tract. Most of the particles of this size will be retained in the upper portions of the respiratory tract, while particles in the range 2 - 5 microns may be deposited in the area of terminal bronchi and alveoli.
Thus, it is apparent that certain particles suspended in an aerosol may be harmful to the respiratory system. In view of this, the present invention was developed. The particles of the present invention are hollow macrospherical particles having a size predominantly between about 10 to 74 microns, and preferably between about 14 and 74 microns, in diameter and having a density greater than 1. These particles are of a large enough size and density to be substantially filtered out by the nose and to avoid deep respiratory tract penetration and deposition.
Prior art particles have either been so small that they have been respired and retained in the lungs, or have been so large that the various valves, dip tubes and orifices of aerosol sprayers become clogged due to agglomeration in the very small openings through which the compositions must pass. This tendency to agglomeration has made it necessary to grind or mill the particles prior to its formulation as a suspension, but even with such prior treatment, agglomeration has continued to be a problem.
It has recently been reported in Drug & Cosmetic Industry, September, 1975, p. 132, that some companies "are attempting to side-step the problem of zirconium inhalation by reformulating their aerosol anti-perspirants to limit spray particles to more than 10 microns, a feat aerosol veterans say may be especially difficult because of shearing and break-up in the upper valve and in the actuator."
Gilman U.S. Pat. No. 3,887,692, issued to the assignee of the present application, disclose basic aluminum halides in microspherical form and aerosol anti-perspirant compositions containing them, as well as processes for preparing the halides in microspherical form. Such microspheres, while uniformly spheroidal in shape, are solid, and thus require more material to make them.
The microspheres disclosed in U.S. Pat. No. 3,887,692 are made by a process in which an aqueous solution of the basic aluminum halides is discharged through a hollow tube or needle in a fine stream against the side of a vortex of whirling organic alcohol. As the vortex whirls, the very fine droplets of the halide assume spherical form. They are then separated from the alcohol and incorporated in an aerosol anti-perspirant composition.
There are several centrifugal particle forming and drying processes and apparatus known in the prior art. See, for example, U.S. Pat. Nos. 1,352,623, 2,043,378, and 3,259,171. The last-mentioned patent discloses a slinger for forming particles to be spray dried. The slinger comprises a screen having a multiplicity of openings through which clay is extruded by centrifugal force, which is used as feed stock. While the patent discloses that the particles are of substantially the same size and shape, they are not hollow nor are they spherical. The particles produced do not have a diameter substantially larger than the openings in the screen.
The above-mentioned patents also disclose apparatus for producing particles by centrifugal force. None, however, teach the use of a porous sintered metal filter capable of producing hollow macrospherical particles in accordance with the present invention.
U.S. Pat. No. 2,829,710 discloses an atomizing dryer but of a substantially different structure than that of the present invention.
Beeco Products Company sells a series of spray head atomizers designated as BEECOMIST spray heads. These devices use controlled-porosity sintered-metal sleeves for spraying fluids, usually liquid solutions for the control of agricultural pests and diseases using droplets from 10 to 1000 microns in diameter. The BEECOMIST spray heads are generally mounted on crop dusting aircraft or carried on farm land vehicles, rather than being placed in a spray drying apparatus.
Conventional spray dryers use an atomizer which is simply a spinning plate which has the liquid solution fed in streams to the underside of the plate. The liquid is spun off the plate by centrifugal force resulting in the formation of liquid droplets which are then dried in mid-air by a hot air stream. See Bulletin 33-3 of Bowen Engineering, Inc., North Branch, N.J. for a description of a conventional spray drying system. Another conventional spray drying apparatus comprises a drilled hole atomizer which includes a cylindrical or basket-like structure for receiving the solution and drilled holes of about 3/16 inch diameter, for example, in the peripheral walls to allow jets of solution to escape into the drying chamber. Both of these conventional methods rely essentially on hydrodynamic centrifugal atomization by the Rayleigh jet break-up phenomenon. Such conventional systems do not produce macrospherical particles such as those disclosed in the present application.